1. Field of the Invention
The present invention relates to an electromagnetic clutch having a magnetic path forming member which faces a friction clutch at the opposite side to an armature for forming a magnetic path circulating from an electromagnetic. It also relates to a method of manufacturing such a magnetic path forming member for an electromagnetic clutch.
2. Discussion of the Related Art
Heretofore, there has been known an electromagnetic clutch of the type that a large-diameter cylindrical member and a small-diameter cylindrical member both made of iron as a magnetic material are welded and integrally joined at their front end portions respectively to the outer and inner surfaces of an annular member made of. stainless steel as a non-magnetic material thereby to constitute a magnetic path forming member. In the electromagnetic clutch of the aforementioned type, the front end surface of the magnetic path forming member is finished by cutting to form a clutch contact surface with which one end of a friction clutch is contactable, and an armature is arranged to face the other side of the friction clutch, while an electromagnet is arranged between the large and small-diameter cylindrical members behind the magnetic path forming member. The magnetic path forming member in the known electromagnetic clutch has the clutch contact surface which is poor in surface flatness for the following reason. That is, stainless steel is hard to cut compared to iron, and the welded portions where stainless steel and iron have been fused have been hardened. Thus, when the iron large and small-diameter cylindrical members and the annular member welded therebetween are cut, the welded portions and the front end portion of the stainless annular member have those portions left uncut and protruded slightly beyond the cut surfaces of the iron large and small-diameter cylindrical members.
To obviate this drawback, Japanese unexamined, published patent application No. 2000-230577 describes a manufacturing method capable of improving the flatness of such a clutch contact surface. In the known method, stepped portions are formed at the inner circumferential edge of a large-diameter cylindrical member as well as at the outer circumferential edge of a small-diameter cylindrical member to retract or recede from the front end surfaces of the both cylindrical members, and an annular member is interposed between the receding stepped portions to define an annular groove. Then, the large and small-diameter members both made of iron are welded to the stainless annular member at the bottom surface of the annular groove, and cutting is then performed on the front end surfaces of the large and small-diameter members only.
However, in the magnetic path forming member described in the aforementioned Japanese patent document, the large and small-diameter cylindrical members are provided at their inner and outer circumferential edges with the stepped portions which recede from the front end surfaces thereof. Thus, the area of the clutch contact surface which is formed by cutting the front end surfaces of the large and small-diameter cylindrical members after the welding to the stainless annular member is decreased by the area corresponding to the annular groove, so that the magnetic path area is diminished disadvantageously. Further, thinning the width in radial direction of the annular member may be conceived of in order to secure a necessary area for the magnetic path. However, this measure cannot be taken for the difficulties in manufacturing the annular member as well as in preventing the magnetic flux from leaking.